1. Field of the Invention
The invention relates to a method for applying solder to printed wiring boards by immersion. The invention also relates to an apparatus for applying solder to printed wiring boards by immersion, having a limiting layer laterally defining the solder deposit, having a retaining device, to which a printed wiring board, prepared for the immersion application of solder, can be attached, a transport device, with which the retaining device is movable with a vertical and/or horizontal component, and a solder container having liquid solder, into which container the retaining device equipped with a printed wiring board can be introduced.
2. The Prior Art
Various methods have become known for applying solder regionally to printed wiring boards in such a way that following the assembly of the boards with SMD components (surface mounted devices), the boards can be electrically and mechanically connected to the components by reflow soldering methods. Details of the reflow soldering method are found for instance in German Patent Application P 37 24 005. In particular, the prior art uses a method in which the solder deposit is applied to the selected regions of the printed wiring board in the form of a deposit of solder by screen printing or mask printing with soldering pastes. In many aspects, this known method cannot be considered optimal. For instance, the investment cost for the screen printing machines, screens and masks required is quite considerable; the screens that must be used wear regularly; the entire screen printing process must be monitored continuously, because it cannot be precluded that the screens used may become plugged in some regions; and so-called thin solder spots are often produced, which can mean a low mechanical and thermal load capacity for the solder connections based on them. Moreover, soldering pastes are expensive, the resolution of the screen printing method is limited, and the optoelectronic positional recognition of assembly systems involves great difficulties, because of soldering paste deposits that do not properly match the intended contour, resulting in deviations in production. The flexibility of interlinked production lines having screen printing robots is greatly impaired, because screen printing machines entail considerable setup times and adjustment work. Moreover, the heating speed of the introduction of heat in reflow soldering with soldering pastes has proved to be limited, because in the heating process, vaporous ingredients and solvents from the soldering paste have to evaporate, which takes a certain amount of time.
A further frequently used method in the prior art is known as immersion application of solder to printed wiring boards; in this method, a prepared printed wiring board is dipped into and removed from a solder bath. After being removed and following a cooling down phase, the metallized regions of the printed wiring board provided with a solder resist means are provided with a solder deposit, which, however, has a more or less convex cross-sectional shape. The height of the solder deposit is also dependent on the dimensions (in the plane of the printed wiring board) of the regions to which solder is to be applied, so that when such regions have different dimensions, variably high solder deposits necessarily result (German Utility Model 86 02 631).
To compensate for this, this document also discloses levelling off the excessively high solder deposits by so-called hot air levelling, that is, the use of an aimed hot-air flow. However, a levelling process of this kind is very labor-intensive, making production more expensive.